Here is a link to a bad ass motion racing sim (PC)
Here is a minimal PS3 sim
and here is another console sim with real car parts
As for the g25/27 steering wheel adaptor, you can find it on ebay. Here are some vids of it, the pedal reverse mod, and the e-brake mod.
and the rest of this guys console race sim
http://www.youtube.com/user/chilicoke#p/u/25/ZNW0uA5xePYIf you just look around, there are all kinds of sim cabinets for racing, you just have to know what you want and go from there. You can go very minimalistic, or way over the top. I like chilicoke's because it is somewhere in between. He has come up with some great mods (ebrake and pedals) for a more authentic controls. Its just like anything else, the sky is the limit, it all depends on how much time and money you want to throw into it. I can see spending about $1300 total for a full setup including parts, TV, console, etc..
Personally, I would copy a setup similar to his, but enclose it a bit more and make it 2 players. Racing games are fun, but just like every other game out there, they are 100x more fun when you have someone to race!
I hope this gives you some good idea's on where to start. Just keep searching other peoples idea's and projects and use what they have done to make something of your own. Good luck!